1. Field of the Invention
The present invention relates to: a developing device which comprises a developing roller and a thickness regulating member that is pressed against the surface of the developing roller so as to regulate the thickness of developer adhered to the surface of the developing roller; and an image forming apparatus comprising the developing device.
2. Description of Related Art
An image forming apparatus for which the electrographic method is employed, such as a laser printer, a digital copying machine, a facsimile or a complex machine, forms an image by electrostatically adsorbing toner adhered to the surface of a developing roller to an electrostatic latent image formed on the surface of a photo conductor so as to develop the electrostatic latent image and transferring the developed electrostatic latent image to recording paper.
Since it is necessary to equalize the amount of toner to be supplied to a development area (development position), where the photo conductor and the developing roller come close, in order to keep high quality of an image to be formed, a doctor blade for regulating the thickness of toner adhered to the surface of the developing roller is pressed against the surface of the developing roller. By rotating the developing roller in a predetermined direction, a redundancy of toner adhered to the surface of the developing roller is scraped off by the doctor blade and a toner layer having a uniform thickness is formed on the surface of the developing roller and is supplied to the development area.
In the structure wherein the doctor blade is pressed against the surface of the developing roller, however, pressure is sometimes applied to toner when toner enters the space between the surface of the developing roller and the doctor blade, and cohesion of toner which is melted by frictional heat sometimes produces a mass of toner. Therefore, there is a problem that development cannot be achieved in a part where a mass of toner enters and toner is not supplied onto the photo conductor, and a white vertical line appears on the formed image. Suggested in order to solve the above problem is a developing device for rotating a developing roller in the backward direction while development is not performed so as to remove toner which has entered the space between the surface of the developing roller and the doctor blade (see Japanese Patent Application Laid-Open No. 61-243473 and Japanese Patent Application Laid-Open No. 4-281478).
Moreover, suggested as a developing device for rotating a developing roller backward is a developing device which rotates the developing roller in the backward direction by a predetermined angle without applying bias voltage having AC component while image formation is not performed and idles the developing roller in the same direction as that for image formation after the rotation in the backward direction, so as to charge toner (see Japanese Patent Application Laid-Open No. 2003-280389).
Moreover, suggested is a developing device which prevents retention of toner at a toner pressing face of a pressing member for regulating the thickness of toner by pressing the pressing member against the circumferential surface of the developing roller while development is not performed and, at the same time, moving the pressing member in the tangential direction thereof, so that the pressing face thereof can be always kept flat (see Japanese Patent Application Laid-Open No. 59-71069).
However, with the developing device in Japanese Patent Application Laid-Open No. 61-243474 which is constructed to rotate the developing roller backward by an angle between a doctor blade for thin film formation and a second doctor blade, the developing roller is rotated approximately half in the backward direction and toner adhered to the developing roller or to the photo conductor may splash to the outside. Moreover, toner which has entered the space between the surface of the developing roller and the doctor blade may not be removed sufficiently by backward rotation of the developing roller.
Moreover, with the developing device in Japanese Patent Application Laid-Open No. 4-281478 wherein the developing roller is rotated in the backward direction for approximately one minute, toner adhered to the developing roller or to the photo conductor may splash to the outside. Moreover, toner which has entered the space between the surface of the developing roller and the doctor blade may not be removed sufficiently by backward rotation of the developing roller.
Moreover, with the developing device in Japanese Patent Application Laid-Open No. 2003-280389 which is intended to charge toner, toner which has entered the space between the surface of the developing roller and the doctor blade may not be removed sufficiently. Furthermore, with the developing device in Japanese Patent Application Laid-Open No. 59-71069 which is constructed to press the pressing member against the circumferential surface of the developing roller while development is not performed and, at the same time, reciprocate the pressing member in the tangential direction thereof, toner adhered to the developing roller may splash to the outside. Moreover, frictional heat generated by reciprocation of the pressing member may melt adhered toner and cause further retention of toner.